I observed the following error when I built VTN (distribution.vtn-manager.karaf).

[INFO] = Installing configuration files from odl-openflowplugin-nsf-services feature
[WARNING] == Installing configuration file
mvn:org.opendaylight.openflowplugin.applications/forwardingrules-manager-config/0.1.0-SNAPSHOT/xml/config

[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 01:05 min
[INFO] Finished at: 2015-04-27T13:27:06+09:00
[INFO] Final Memory: 89M/775M
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal org.apache.karaf.tooling:karaf-maven-plugin:3.0.3:install-kars (process-resources) on project distribution.vtn-manager.karaf: Can not install odl-vtn-manager-all/0.3.0-SNAPSHOT feature: Illegal character in scheme name at index 0:
[ERROR] mvn:org.opendaylight.openflowplugin.applications/forwardingrules-manager-config/0.1.0-SNAPSHOT/xml/config
[ERROR] -> [Help 1]

This was caused by an illegal <configfile> element in features.xml.
Karaf never removes leading and trailing whitespace characters in <configfile> value. So <configfile> has to be written without leading and trailing whitespace characters.